Mux is a character backend (host side) device, which multiplexes multiple frontends with one backend device. The following is a few lines from the QEMU manpage [1]:
A multiplexer is a "1:N" device, and here the "1" end is your specified chardev backend, and the "N" end is the various parts of QEMU that can talk to a chardev. But sadly multiple backends are not supported. This work implements multiplexing capability of several backend devices, which opens up an opportunity to use a single frontend device on the guest, which can be manipulated from several backend devices. The motivation is the EVE project [2], where it would be very convenient to have a virtio console frontend device on the guest that can be controlled from multiple backend devices, namely VNC and local TTY emulator. The following is an example of the QEMU command line: -chardev mux-be,id=mux0 \ -chardev socket,path=/tmp/sock,server=on,wait=off,id=sock0,mux-be-id=mux0 \ -chardev vc,id=vc0,mux-be-id=mux0 \ -device virtconsole,chardev=mux0 \ -vnc 0.0.0.0:0 which creates 2 backend devices: text virtual console (`vc0`) and a socket (`sock0`) connected to the single virtio hvc console with the backend multiplexer (`mux0`) help. `vc0` renders text to an image, which can be shared over the VNC protocol. `sock0` is a socket backend which provides biderectional communication to the virtio hvc console. Once QEMU starts VNC client and any TTY emulator can be used to control a single hvc console, for example these two different consoles should have similar input and output due the buffer multiplexing: # VNC client vncviewer :0 # TTY emulator socat unix-connect:/tmp/sock pty,link=/tmp/pty tio /tmp/pty v3 .. v4: * Rebase on latest chardev changes * Add unit tests which test corner cases: * Inability to remove mux with active frontend * Inability to add more chardevs to a mux than `MUX_MAX` * Inability to mix mux-fe and mux-be for the same chardev v2 .. v3: * Split frontend and backend multiplexer implementations and move them to separate files: char-mux-fe.c and char-mux-be.c v1 .. v2: * Separate type for the backend multiplexer `mux-be` * Handle EAGAIN on write to the backend device * Support of watch of previously failed backend device * Proper json support of the `mux-be-id` option * Unit test for the `mux-be` multiplexer [1] https://www.qemu.org/docs/master/system/qemu-manpage.html#hxtool-6 [2] https://github.com/lf-edge/eve Roman Penyaev (8): chardev/char: rename `MuxChardev` struct to `MuxFeChardev` chardev/char: rename `char-mux.c` to `char-mux-fe.c` chardev/char: move away mux suspend/resume calls chardev/char: rename frontend mux calls chardev/char: introduce `mux-be-id=ID` option chardev/char-mux: implement backend chardev multiplexing tests/unit/test-char: add unit test for the `mux-be` multiplexer qemu-options.hx: describe multiplexing of several backend devices chardev/char-fe.c | 25 ++- chardev/char-mux-be.c | 290 ++++++++++++++++++++++++ chardev/{char-mux.c => char-mux-fe.c} | 157 +++++-------- chardev/char.c | 133 +++++++++-- chardev/chardev-internal.h | 55 ++++- chardev/meson.build | 3 +- include/chardev/char.h | 8 +- qapi/char.json | 31 ++- qemu-options.hx | 78 +++++-- system/vl.c | 4 +- tests/unit/test-char.c | 306 +++++++++++++++++++++++++- 11 files changed, 922 insertions(+), 168 deletions(-) create mode 100644 chardev/char-mux-be.c rename chardev/{char-mux.c => char-mux-fe.c} (71%) Signed-off-by: Roman Penyaev <r.peni...@gmail.com> Cc: "Marc-André Lureau" <marcandre.lur...@redhat.com> Cc: qemu-devel@nongnu.org -- 2.34.1
